I know what you mean. TMP came out before I was born, and I always knew that theme as the TNG theme. I can even remember watching Star Trek V and thinking "They just re-used the TNG theme; how lame!" only to discover later that TNG was really the one that did the re-using.
Now, I just appreciate the TMP theme as the much grander sounding original version of the TNG theme; the way it was meant to be heard, similar to the way the theme from the Stargate movie sounds so much better than the SG-1 theme.
But I never had the experience of hearing the TMP theme before TNG existed, so I can't totally separate the two in my mind. (An even weirder experience was seeing an NFL highlight reel from the early 1980s that used the TMP theme in its soundtrack. At the time, I'm sure they chose it as an energetic movie theme, but now it's funny to hear that music as the soundtrack for football!)
Well, you're a youngin' aren't cha!
I've loved movie music since I was a kid, and my Sci_Fi music was my all time favorite. I can remember when I sat down to watch the first episode of TNG as it began airing in syndication, how excited I was at the prospect of getting new theme music to add to the collection of sound that had come before. Boy did I feel gypped. Later, when I heard the Dennis McCarthy music, I didn't feel quite so bad. I eventually came to appreciate the re-use, but it will never hold the same reverence in my heart as the TMP original does. McCarthy's version isn't horrible, and I've since committed it to memory, but it does have a touch of the corny. The hints you can hear of it in rest of the pilot soundtrack are better.
I have to admit, I was about to ask A beaker full of death who the devil Lars Gnuderstardt was!